1. Field of the Invention
This invention relates to forming smooth aluminum films, and more particularly, to a method of depositing aluminum having a subphase of aluminum nitride to produce a hillock-free aluminum film.
2. Description of the Related Art
Metallic films are commonly used to form interconnects on integrated circuits and for display devices such as field emission displays (FEDs). Aluminum is a popular material choice for such films because of its low resistivity, adhesion properties, and mechanical and electrical stability. However, aluminum also suffers from process-induced defects such as hillock formation which may severely limit its performance.
Hillocks are small nodules which form when the aluminum film is deposited or subjected to post-deposition processing. For example, hillocks can result from excessive compressive stress induced by the difference in thermal expansion coefficient between the aluminum film and the underlying substrate used during post-deposition heating steps. Such thermal processing is typical in the course of semiconductor fabrication. Hillock formation may create troughs, breaks, voids and spikes along the aluminum surface. Long term problems include reduced reliability and increased problems with electromigration.
Hillocks may create particularly acute problems in the fabrication of integrated FED and similar devices. Many FEDs comprise two parallel layers of an electrically conductive material, typically aluminum, separated by an insulating layer to create the electric field which induces electron emission. The insulating film is deliberately kept thin (currently about 1-2 μm), to increase the field effect. Hillock formation in the underlying aluminum layer may create spikes through the insulating layer, resulting in a short circuit and complete failure of the device.
Some efforts have been made to reduce or prevent the formation of hillocks in aluminum films. For instance, alloys of aluminum with Nd, Ni, Zr, Ta, Sm and Te have been used to create aluminum alloy thin films which reduce the formation of hillocks. These alloys, however, have been unsatisfactory in producing low resistivity metal lines while still avoiding hillock formation after exposure to thermal cycling.
Accordingly, there is a need for a smooth aluminum film having low resistivity suitable for integrated circuit and field effect display technologies. In particular, the aluminum film should remain hillock-free even after subsequent thermal processing.